Conventionally, each of electrophotographic image forming apparatuses (printing apparatuses) such as copying machines, printers and facsimiles is provided with a photoconductor drum, a charge device, an exposure device, a developing device, a transfer device, a fixing device, and the like.
In the image forming apparatus, the exposure device performs exposure with respect to a photosurface (electrostatic latent image face) of the photoconductor drum charged by the charge device so as to form an electrostatic latent image. In the developing device, the formed electrostatic latent image is developed with toner (developer) so as to form a toner image (visible image). The toner image is transferred to a sheet (recording material; printing medium such as a standard paper or an OHP sheet), and then fixed by the fixing device.
In the image forming apparatus, density of toner used for development has an influence on quality of a formed image. Therefore, in conventional image forming apparatus, in order to always acquire images having constant density, the density of the toner is detected, and when the density of the toner drops, toner is supplied, thereby controlling the density of the toner in a suitable range.
For example, each of Japanese Laid-Open Patent Publication No. 215575/1986 (Tokukaishou 61-215575) (published on Sep. 25, 1986) and Japanese Laid-Open Patent Publication No. 207214/1998 (Tokukaihei 10-207214) (published on Aug. 7, 1998) discloses a technique in which an amount of toner supply is controlled according to a result of detection carried out by a toner density sensor.
Further, Japanese Laid-Open Patent Publication 304486/1992 (Tokukaihei 4-304486) (published on Oct. 27, 1992) discloses a technique in which a toner consumption ratio is calculated based on (i) a printing ratio obtained from pixel data to be printed and (ii) page number information, and toner is supplied based on the thus calculated toner consumption ratio. Further, Japanese Laid-Open Patent Publication 304486/1992 discloses a technique in which a cumulative error of the calculated toner consumption ratio is corrected through judgment on a threshold value of an output from a toner density detection device.
Further, Japanese Laid-Open Patent Publication No. 214672/2000 (Tokukai 2000-214672) (published on Aug. 4, 2000) discloses a technique in which: in a case where the printing ratio obtained from the pixel data is large, toner supply is controlled so that the output from the toner density sensor is a standard value, and in a case where the printing ratio obtained from the pixel data is small and the output from the toner density sensor is small, when the density of a patch image detected by an optical sensor is not less than a standard value, toner is intermittently supplied, and when the density of the patch image is not more than the standard value, toner is continually supplied.
Further, Japanese Laid-Open Patent Publication No. 151375/2004 (Tokukai 2004-151375) (published on May 27, 2004) discloses a technique in which the threshold value of the output from the toner density sensor is corrected based on the printing ratio obtained from the pixel data so as to control the toner supply.
However, as in the techniques disclosed in Japanese Laid-Open Patent Publication No. 215575/1986, Japanese Laid-Open Patent Publication No. 207214/1998, and Japanese Laid-Open Patent Publication No. 151375/2004, when toner supply is controlled based on the result of detection carried out by the toner density sensor, it takes time to actually supply toner after detecting the toner density, and accordingly supply operation is delayed. Therefore, toner density tends to be insufficient.
Further, as in the techniques disclosed in Japanese Laid-Open Patent Publication 304486/1992 and Japanese Laid-Open Patent Publication No. 214672/2000, when the amount of toner supply is controlled based on the printing ratio obtained from the pixel data to be printed, there is a case where, for example, an amount of supplied toner is different from an amount that is actually needed because of a difference between the amount of toner consumption based on the calculated printing ratio and an amount of actual toner consumption. Particularly, when toner is excessively supplied, the only way to reduce an amount of the excessively supplied toner is to consume toner by printing so that toner density is optimized. Therefore, quality of an image developed on a recording material inevitably drops.